1. Field of the Invention
This invention relates to a sample-and-hold circuit having a function of compensating for excess signal charge and a liquid crystal display apparatus constituting a driving circuit by use of the sample-and-hold circuits.
2. Description of the Related Art
A liquid crystal display apparatus includes a display unit having liquid crystal cells arranged in a matrix form and a plurality of signal lines supplied with image signals and intersected with a plurality of scanning lines, a display driving circuit for sampling and supplying image signals to the signal lines, and a scanning line selection circuit for selectively driving the scanning line. The display driving circuit is mainly constructed by a sample-and-hold circuit.
The display driving circuit of the conventional liquid crystal display apparatus includes sample-and-hold circuits of a number corresponding to the number of picture cells necessary for constituting one horizontal line and a shift register for supplying sampling pulses to the sample-and-hold circuits. Each sample-and-hold circuit includes a first MOS transistor serving as an analog switch for sampling a video signal and a holding capacitor for holding the sampled signal charge. The terminal voltage of the capacitor is derived from an output terminal connected to the signal line via an output buffer controlled by an output enable signal, and a second MOS transistor of charge compensation for releasing excess charges exceeding an amount of necessary signal charges which correspond to the video signal and which are included in the charges stored in the capacitor is connected between the capacitor and the output buffer. The excess charges may include channel charges stored in the channel when the first MOS transistor is set in the ON state and moved into the source or drain when it is turned off, or feed-through charges entering from the gate via the coupling capacitor. The drain and source of the second MOS transistor are connected to each other to make a capacitor and the gate thereof is controlled by a signal obtained by inverting a sampling pulse by means of an inverter.
For example, the sample-and-hold circuit having the above-described function of compensating the excess signal charge is analyzed in the article by Roubik Gregorian et al. in "Analog MOS Integrated Circuits for Signal Processing" pp 469 to 470.
Particularly, in a case where the conventional liquid crystal display driving circuit having the above-described excess signal charge compensating function is formed in an IC form, the output potential of each sample-and-hold circuit will vary by variation in the element characteristics, causing the image quality to be deteriorated.
That is, when a sampling pulse is supplied to the first MOS transistor and a sampling pulse inverted by an inverter is supplied to the second MOS transistor for compensation so as to sample an input signal, the time at which the second MOS transistor for compensation is turned on varies according to a variation in the delay time of the inverter. For example, if the second MOS transistor is turned on early, charges are directly supplied from the video signal line into the channel of the second MOS transistor because the first MOS transistor is still kept on. In contrast, if the second MOS transistor is turned on later than normal, part of the charges already stored in the charge-holding capacitor is supplied into the channel of the second MOS transistor. This means that the effect of the second MOS transistor for charge compensation, that is, the effect of removing the excess charges such as the channel storage charges of the first MOS transistor or feed-through charges from the gate will vary. As a result, the output potential will vary even when the input potential of each sample-and-hold circuit is kept unchanged.
As described above, in the conventional liquid crystal display driving circuit, particularly when it is formed in an IC form, the output potential varies according to a variation in the element characteristic of each sample-and-hold circuit, thereby deteriorating the quality of the displayed image.